Existing wires utilized for wire saws may typically be made of relatively high tensile steel which may be deep drawn down to achieve relatively fine wire diameters in the range of 140 to 380 μm. The lower limit in wire diameter may be limited by the number and practicality of stages of conventional wire drawing, and the ability to achieve significant levels of ductility which are reduced from work hardening. Wire cutting saws may include two different varieties, such as slurry abrasive or diamond wire saws. In slurry abrasive wire cutting, a bare steel wire or brass coated steel wire may be utilized in combination with a slurry abrasive which may include a relatively large variety of abrasives such as SiC. The relatively fast moving wire may contact the abrasive in the liquid slurry, which may become trapped between the wire and the substrate resulting in the cutting of the substrate. In diamond wire cutting, a steel wire may be used as the wire base, which is then coated with an electrolytic copper sheet impregnated with diamonds, and 10 to 120 μm in size. The entire wire may then be coated with a nickel overstrike to reinforce the wire. As may be appreciated the steel base wire is one factor limiting the total wire diameter and the impregnated copper and nickel coatings add to the diameter. There may be several advantages and disadvantages between the slurry abrasive wire and diamond wire cutting techniques. For wafer cutting, a diamond coated wire can offer advantages such as the precision of the cut compared to slurry abrasion cutting which may wander. Alternately, slurry abrasive cutting offers an advantage with respect to lower edge chipping compared to diamond wire cutting and, accordingly, slurry abrasive wire cutting appears to be used prevalently in cutting large diameter silicon ingots.
For any high value material including silicon, germanium, gallium arsenide, quartz, glass, etc., the material losses or kerf losses during cutting may be significant. One overriding factor in total kerf loss during cutting may be the wire diameter utilized, wherein smaller wire diameters may lead to lower kerf losses. The following case example regarding silicon wafer illustrates the value of these losses for silicon in the microelectronics and photovoltaic industries.
That is, one key cost factor for silicon wafer processing may include the material lost during cutting or kerf losses. As the price of raw materials has increased and the thickness of the wafer has decreased, the kerf loss has been an increasingly important factor. With current wire technology it has been estimated that the kerf thickness loss may ultimately be brought down to 150 μm in thickness. Furthermore, this loss becomes increasingly important as wafer size decreases. For example, for industrial solar cells, in 2004 the average thickness was 330 μm but by 2007, the average wafer thickness was 210 μm. Additionally, the recycling of silicon kerf is challenging since it is exists in a slurry with polyethylene glycol liquid containing impurities including iron from the wire and SiC abrasives.
In 2006, the world wide production capacity of polysilicon was at 37,500 tons. It has been estimated that 70% of all polysilicon feedstock ends up as usable silicon ingot resulting in 26,250 tons produced. The average kerf loss in wafer sawing process is estimated to be 35% which results in a total silicon waste at 9,188 tons. In 2006, the average price per pound of silicon varied widely depending on the type with the following values published; Solar Poly Price at $36.3/lb, Semiconductor CZ Price at $27.21/lb, Semiconductor FZ Price at $90.70/lb and Spot Market Price depending on availability at $136.05/lb. A conservative estimate based on prices above is a cost basis of $55/lb for value of microelectronic grade silicon. Thus, the yearly monetary value of kerf waste can be estimated at $1.01 Billion dollars per year. Furthermore, manufacturing of microelectronic grade silicon is relatively energy intensive and involves high temperatures at extended times in order to extract, purify, and grow crystals from the melt. It has been estimated that electron energy usage is 90.7 MW hours per ton of silicon ingot. The average kerf loss in the wafer sawing process as stated earlier is 9,188 tons. Thus, the total energy lost for wasted silicon is 833,352 MW hours. Considering a rough estimate of the average cost of electricity at $10.00 per MW hour, then the total wasted electricity cost is $0.83 billion dollars per year.